Star Cluster - Stellar Associations

Stellar Associations

Once an open cluster has become gravitationally unbound, or if a newly formed group of stars fails to form a cluster, the constituent stars will continue to move on similar paths through space. The group is then known as a stellar association, or a moving group. Most of the stars in the Big Dipper are members of a former open cluster, the Ursa Major Moving Group, and have similar proper motions. Other stars across the sky, including Alphecca and Zeta Trianguli Australis, are related to this group. The Sun lies at the edge of this stream of stars at the moment, but isn't a member as is shown by its different galactic orbit, age, and chemical composition.

Another stellar association is that surrounding Mirfak (α Persei), which is very prominent in binoculars. Distant moving clusters cannot readily be detected since the proper motions of the stars need to be known.